1. Field of Invention
This invention relates to processes for the dehydrogenation of hydrocarbon feed streams, particularly C3 to C5 alkane hydrocarbon feed streams, using a particular chromia-eta alumina catalyst. The invention more specifically relates to processes for the dehydrogenation of light alkane hydrocarbon feed streams using a stabilized chromia alumina catalyst containing a zirconium additive, wherein the alumina is eta alumina.
2. Prior Art
Alkane dehydrogenation is a recognized process for production of a variety of useful hydrocarbon products, such as isobutylene for conversion to MTBE, isooctane and alkylates to supplement and enrich gasolines and propylene for use in the polymer industry. There are several current catalytic processes useful for catalytic dehydrogenation of light alkanes, including the Süd-Chemie CATOFIN® process, the Linde/BSF process, UOP's Oleflex® process, Phillips' Star™ process and the Snamprogetti-Yarsintee process. The catalysts that are used in these processes are manufactured from two different groups of materials. The Süd-Chemie CATOFIN® process, the Linde/BSF process and the Snamprogetti-Yarsintee process utilize chromia-alumina catalysts. In contrast, the catalysts for the UOP and Phillips processes comprise precious metal(s) on support catalysts, as disclosed for example in U.S. Pat. Nos. 4,880,764, 4,786,625 and 4,716,143.
Chromia-alumina dehydrogenation catalyst technology has been in use for over fifty years as disclosed in U.S. Pat. Nos. 2,423,029, 2,945,823, 2,956,030, 2,985,596, 2,399,678 and GB 942,944. In particular, GB 942,944 discloses a dehydrogenation catalyst for the dehydrogenation of aliphatic hydrocarbons having three to five carbon atoms. The catalyst disclosed by the GB '944 patent was prepared by dehydrating an aluminum trihydrate composition comprising 60 to 100 percent beta alumina trihydrate, heating the resulting dehydrated alumina with steam to adjust its surface area to a range of 100 to 200 m2/g, depositing from about 10 to about 25 percent of Cr2O3 onto the resulting alumina carrier and steam treating the resulting catalyst at an elevated temperature.
Other chromia-based dehydrogenation catalysts are disclosed in U.S. Pat. Nos. 3,719,721, 4,746,643 and 5,378,350.
With dehydrogenation catalysts used for the above processes, stability of the catalyst plays an important role in the overall efficiency of the dehydrogenation process. Because of the extreme temperature ranges at which the catalytic dehydrogenation procedure is conducted, the life expectancy of the catalyst is often limited. Thus, improving the thermal stability of the catalyst translates into longer catalyst life, allowing for better catalyst utilization and ultimately results in lower consumption of the catalyst during the dehydrogenation process.
One proposed method of stabilizing chromia-alumina dehydrogenation catalysts is by the addition of zirconia as disclosed in U.S. Pat. No. 2,374,404.
In addition, U.S. Pat. No. 2,943,067 discloses an alkali metal promoted, chromia-alumina catalyst with improved stability, wherein the alumina carrier support is derived from a gel alumina. The '067 patent claims improved performance of its aged catalyst for the production of butadiene over the commercially available Harsaw catalyst, which catalyst is produced using a Bayer process alumina for forming the support.
Another group of materials that are used to stabilize an alumina support for dehydrogenation catalysts is siliceous compounds, such as those disclosed in U.S. Pat. No. 2,956,030.
There are a number of different types of alumina that are available for use as the support for dehydrogenation catalysts. However, mid to high surface area, gamma alumina has consistently been the preferred choice as the carrier for such catalysts as disclosed, for example, in U.S. Pat. Nos. 2,956,030, 2,945,823 and 2,374,404.
In particular, gamma alumina is preferred over eta alumina as the carrier material for dehydrogenation catalysts. For example, in Tsuchida, et al., “The effect of Cr3+ and Fe3+ ions on the transformation of different aluminum hydroxide to alpha-Al2O3”, Thermochimica ACTA, 64, pages 337-353 (1983), the preference for gamma alumina over eta alumina is clear. The article states that during the formation of alpha alumina containing Cr3+ ions, the transformation of bayerite containing chromium ions from eta alumina to alpha aluminum was “accelerated.” In contrast, the transformation of boehmite containing chromium ions from gamma alumina to alpha alumina was “inhibited.” Acceleration of this transformation to alpha alumina, as is exhibited by eta alumina, results in reduced stabilization of the catalyst while inhibition in the transformation, as is exhibited by gamma alumina, enhances stabilization of the catalyst end product.
The preference for gamma alumina as the support material for catalysts in general, especially where enhanced stability at higher temperatures is required, is also discussed in Richardson, James T.; Principles of Catalyst Development, (1989). The preference for gamma aluminum as a support material is specifically discussed at pages 35 and 36, especially in a situation where a small quantity of zirconia is added to the alumina to stabilize the catalyst.
Another example of the preference for gamma alumina over eta alumina as the material used to form the carrier of a dehydrogenation catalyst is disclosed in U.S. Pat. No. 2,943,067. At column 5, Example 1, the performance of an alumina supported catalyst produced by the Bayer process (which produces a gamma alumina) is described as being superior to a catalyst prepared from a gel-type alumina, which upon heating normally converts to an eta alumina. (Alumina produced by the Bayer process produces gibbsite, which upon heating converts to gamma alumina.) Thus, the '067 patent teaches the superiority of gamma alumina over eta alumina as the carrier for dehydrogenation catalysts.
The lack of thermal stability for catalyst produced from eta alumina is also discussed in Oberlander, Richard K.: Aluminas for Catalysts—Their Preparation and Properties, page 69 (1983).
This preference for gamma alumina over eta alumina for catalysts is not surprising because gamma alumina is generally perceived as having a greater thermal stability over eta alumina. In fact, gamma alumina has become the standard alumina utilized for dehydrogenation catalysts. (The market has accepted this principle as gamma alumina is readily available in the market while eta alumina is sparsely available, if at all.)
Although dehydrogenation catalysts prepared from chromia-alumina catalysts have been extensively employed for many years, there are still problems with current catalysts, especially their thermal stability. Even when these catalysts are stabilized by the addition of an additive, such as a zirconium or a silicon compound, these catalysts still show limited stability because of the severity of the operating conditions, particularly the high temperature, during the dehydrogenation procedure.
Accordingly, it is an object of the invention to disclose processes for the dehydrogenation of hydrocarbon feed streams, particularly aliphatic hydrocarbon feed streams, wherein the hydrocarbons preferably contain 3 to 5 carbon atoms, utilizing an improved chromia-alumina catalyst, wherein the alumina utilized in the catalyst is eta alumina.
These and other objects can be obtained by the processes for the dehydrogenation of hydrocarbons, particularly aliphatic hydrocarbons, which is disclosed by the present invention.